[libata] fix oops on non-DMA bmdma hardware

Alan noted: "bmdma may be zero but the bmdma_irq_clear function gets
called even in this case during pure PIO operation. Check we have a
bmdma before we use it."

I fixed this by adding a check for zero.  While was I there, I fixed the
non-standard indentation of the small function's code.

Signed-off-by: Jeff Garzik <jeff@garzik.org>